As the major causative agent of swine viral diarrhea, porcine epidemic diarrhea virus (PEDV) has caused massive losses to the economies of swine raising countries. Accordingly, the serological detection of corresponding antibodies would be beneficial to diagnose PEDV indirectly to control the disease. In this study, an indirect enzyme-linked immunosorbent assay (ELISA) based on the recombinant truncated spike (S) protein of PEDV was developed and validated. The reaction conditions of the developed indirect ELISA were optimized. This indirect ELISA was compared to indirect immunoinfluscent assay (IFA), and the overall coincidence rate was 96.74% based on testing 368 clinical serum samples with different PEDV antibody levels. No cross-reactivity with other common swine pathogens was detected for the developed S1 indirect ELISA. Finally, the S1 indirect ELISA was applied to detect serum antibodies of 3304 field samples collected from different pig farms in eastern China, and it presented an overall substantial agreement on the PEDV infection status. This established S1 indirect ELISA is capable of detecting serum antibodies against PEDV, and due to its high sensitivity and specificity, it could be applied for serological evaluation and indirect diagnosis of PEDV infection.

Glaesserella parasuis (G. parasuis) is a commensal bacterium in the upper respiratory tract of pigs that can also cause the swine Glässer disease, which induces an intensive inflammatory response and results in significant economic losses to the swine industry worldwide. G. parasuis can cause disease through infection of the respiratory tract, resulting in systemic infection, but the mechanism is largely unknown. Recently we showed that Glaesserella parasuis serotype 4 (GPS4) increased swine tracheal epithelial barrier permeability, resulting in easier bacterial translocation. Tight junction proteins (TJ) play a crucial role in maintaining the integrity and impermeability of the epithelial barrier. GPS4 decreased the expression of the TJ ZO-1 and occludin in swine tracheal epithelial cells (STEC). Furthermore, the proinflammatory cytokines IL-6, IL-8 and TNF-α were significantly upregulated in GPS4-infected STEC, and both the MAPK and NF-κB signaling pathways were activated and contributed to the expression of TNF-α. We demonstrate that the production of proinflammatory cytokines, especially TNF-α, during GPS4 infection was involved in barrier dysfunction. Additionally, animal challenge experiments confirmed that GPS4 infection downregulated TJ in the lungs of piglets and induced a severe inflammatory response. In general, G. parasuis infection downregulated the expression of TJ and induced massive secretion of proinflammatory cytokines, resulting in epithelial barrier disruption and favoring bacterial infection. This study allowed us to better understand the mechanism by which G. parasuis crosses the respiratory tract of pigs.

Porcine circovirus type 2 (PCV2) is considered as the primary pathogen of porcine circovirus-associated disease (PCVAD), which results in significant economic losses worldwide. Clinically, PCV2 often causes disease through coinfection with other bacterial pathogens, including Streptococcus suis (S. suis), and especially the highly prevalent S. suis serotype 2 (SS2). The present study determined that continuous PCV2 infection in piglets down-regulates tight junction proteins (TJ) ZO-1 and occludin in the lungs. Swine tracheal epithelial cells (STEC) were used to explore the mechanisms and consequences of disruption of TJ, and an in vitro tracheal epithelial barrier model was established. Our results show that PCV2 infection in STEC decreases the expression levels of ZO-1 and occludin and increases the permeability of the tracheal epithelial barrier, resulting in easier translocation of SS2. Moreover, Western blot analysis indicates that PCV2 infection activates the JNK/MAPK pathway. The disruption of TJ in SETC and increased permeability of the epithelial barrier induced by PCV2 could be alleviated by inhibition of JNK phosphorylation, which indicates that the JNK/MAPK pathway regulates the expression of ZO-1 and occludin during PCV2 infection. This study allows us to better understand the mechanisms of PCV2 coinfection with bacterial pathogens and provides new insight into controlling the occurrence of PCVAD.